Is a post always placed when one has a crown put on for a rct. how is the post put in & what are the risk?

Almost always: When a tooth needs root canal therapy it is usually because of extensive decay. After cleaning out the decay and accessing the center of the tooth for the root canal therapy, there's not much left of the tooth for the crown to hold onto. A post (or posts) are placed together with a core. The post and core translate the chewing forces on the crown down to the roots that are still strong.